The present invention relates to fuel cells, and more particularly, to a fuel cell system that has an improved purge means for the containment vessel and for the cathode oxidants.
In recent years, fuel cell power plant systems are known as a method for the direction conversion of the energy, that fuel has, into electrical energy. Such fuel cell powerplant systems normally have fuel cell unit cells configured by an electrolytic material being sandwiched between a pair of porous electrodes. Fuel such as hydrogen or the like is brought into contact with the rear of the anode which is one of the electrodes, while an oxidizing agent such as air or the like is brought into contact with the rear of the cathode which is the other electrode and the electro-chemical reaction that occurs when this is done is utilized to take electrical energy from between the two electrodes. Such a unit fuel cell is configured from a fuel cell stack that consists of many laminated layers, and it is possible to take electrical energy at high efficiency for as long as the fuel and the oxidizing agent are supplied to the stack.
In addition, in order for this electro-chemical reaction to have a favorable efficiency under conditions of high temperature and high pressure, the general practise is to house the fuel cell stack inside a sealed containment vessel, to perform operation under high pressure while cooling water is introduced to a cooler that is provided inside the fuel cell stack, and for a certain set of high temperature operating conditions to be maintained.
FIG. 6 is a view of such a conventional type of fuel cell system. More specifically, a fuel cell stack 1d comprising an anode 1a, a cathode 1b and cooling apparatus 1c is housed inside a containment vessel 1e to configure the fuel cell main unit 1. The structure of the fuel cell stack 1d is a laminate of many plates of unit cells such as has been described above but it shown as only a single element in the figure, for the sake of simplification.
When such a fuel cell is generating power, fuel such as highly concentrated hydrogen gas or the like is supplied from the fuel supply apparatus 2 to the anode 1a via a fuel valve 3. In addition to this, an oxidizing gas such as air or the like (air in the case of the following example) is supplied from an air supply apparatus 4 to the cathode 1b via an air valve 5. Furthermore, cooling water is supplied via a cooling water loop to the cooling apparatus 1c of the fuel cell stack 1d and is discharged from the cooling apparatus 1c when it has absorbed the heat that is generated by the fuel cell. The high-temperature water or the high-temperature two-phase flow that is discharged is led to a steam separator 6 and is separated into a steam 6a and water 6b. Also, in the case when there is high-temperature water at the outlet of the cooling apparatus 1c of the fuel cell stack 1d, there is a suitable pressure reduction means provided between the cooling apparatus 1c and the steam separator 6 so that it is possible to separate the steam 6a inside the steam separator 6. The steam 6a is sent to a heat recovery system 7 where the heat is used for various purposes. On the other hand, the water 6b, that has been separated from the steam again enters the cooling water loop, has its temperature adjusted at the heat exchanger 8 and is led to the cooling apparatus 1c of the fuel cell stack 1d by the cooling water pump 9. Also, insufficiency of the cooling water that is discharged as steam are replenished with water supplied from outside from a feed water system 10 via a feed water valve 11. Furthermore, an inert gas such as nitrogen or the like is supplied from an inert gas supply apparatus 12 and via an inert gas supply valve 13 to inside the containment vessel 1e so as to purge inside the containment vessel. After this, the inert gas such as nitrogen or the like is released to the outside from a purge discharge line 14.
The following is a description of the purposes of purging inside the containment vessel. More specifically, when the fuel cell stack is manufactured, the anodes and the cathodes of each of the laminated fuel cell units 1 are made to have sufficient gas sealing but deterioration with time accompanying extended periods of use or some other cause of sharp changes in the operating pressure or the like may cause some amount of gas leakage to occur while the cell 1 is in operation. In cases such as this, the fuel gas and air becomes mixed and are retained inside the containment vessel 1e, and there is a danger that there may be an explosion if there are the conditions for detonating gas. In order to remove this danger, inert gas is used to continuously or periodically purge the containment vessel 1e.
However, in such a conventional fuel cell powerplant system having the configuration described above, there are the following problems that should be solved, More specifically, in a large-scale powerplant system that has a stack having many fuel cells, when purging of the containment vessel of each stack is continuously or periodically performed, the amount of nitrogen gas or the inert gas consumed becomes considerably large when long-term operation is considered, and the running cost of the powerplant system overall is made large. In addition, it is necessary to have an initial investment for the apparatus (such as evaporators for the liquid nitrogen, and the like, for example) for the supply and storage of the inert gas and so this is disadvantageous for both the facility cost and the facility space, and it is required that there be the development of a means for purging the containment vessel and which does not use an inert gas such as nitrogen or the like.
The present invention is proposed in order to eliminate these problems and an object of the present invention is to enable the purging of the containment vessel of a fuel cell by a less expensive means, and to provide a fuel cell powerplant system that requires a smaller installation space.
Furthermore, as shown in FIG. 7, in a conventional fuel cell powerplant system, the configuration is such that an inert gas such as nitrogen or the like is introduced from the inert gas supply apparatus 12, to the inlet side of the cathode 1b and via an inert gas supply valve 13. This is so that the inert gas can be inserted into the cathode during a low load operation of the powerplant, or immediately after the shut down of powerplant operation and so that the concentration of the oxygen in the cathode can be reduced when there is an excessive voltage generated by the fuel cell.
The purpose of reducing the oxygen concentration inside the cathode as described above, is as follows. More specifically, when the fuel cell performs power generation operation at a low current density or when the powerplant stops its powergenerating operation, the cell voltage becomes higher than that to the rated powerplant operation generates when there is still sufficient air remaining inside the cathode. If this voltage exceeds a predetermined level and the status of excess voltage continues, then this is likely to facilitate a deterioration of the fuel cell performance. Therefore, in order to prevent such deterioration, the oxygen concentration inside the cathode is reduced in accordance with necessity so as to prevent the generation of an excessive cell voltage.
However, in a conventional fuel cell powerplant system having the configuration such as has been described above, there is the following problem that still has to be solved. More specifically, in a large-scale power plant system that has many fuel cell stacks, reducing the concentration of the oxygen inside the cathodes of each stack must be performed immediately after the shut down of powerplant operation or when there is low-load operation and so the amount of the nitrogen gas or inert gas used is considerable when there is operation for an extended period and when there is frequent start-up and shut-down, and the running cost of the powerplant system as an entirety becomes large. In addition, it is also necessary to have an initial investment for the apparatus (such as evaporators for the liquid nitrogen, and the like) for the storage and supply of the inert gas and so this is disadvantageous in terms of the facility cost and the installation space. It is therefore desirable that there be the development of a means for reducing the concentration of the oxygen at the fuel cell cathode and that does not use an inert gas such as nitrogen.
Furthermore, the present invention is proposed to eliminate these problems described above, and an object of it is to provide a fuel cell powerplant system that reduces the concentration of the oxygen at the fuel cell cathode by a less expensive means, and that enables the installation space for the fuel cell to be made smaller.